1. Field of the Invention
This invention relates to an image forming apparatus for forming an electrostatic latent image on an image bearing member by electrophotography and visualizing the electrostatic latent image by means of a developer and also to a method of detecting the amount of residual developer. More particularly, the present invention relates to an electrophotographic image forming apparatus having means for detecting the amount of residual developer adapted to iteratively detect the amount of residual developer contained in the developing apparatus or the process cartridge of the image forming apparatus and also to a method of detecting the amount of residual developer.
For the purpose of the present invention, electrophotographic image forming apparatus include electrophotographic copying machines, electrophotographic printers (LED printers, laser beam printers and so on) and electrophotographic facsimile machines. For the purpose of the present invention, a process cartridge refers to a combination of at least a charging means, a developing apparatus or a cleaning means and an electrophotographic photosensitive member that operates as an image bearing member integrally put into a cartridge, which cartridge is detachably mountable to the electrophotographic image forming apparatus main body. A process cartridge refers at least to a combination of a developing apparatus and an electrophotographic photosensitive member that are integrally put into a cartridge, which cartridge is detachably mountable to the electrophotographic image forming apparatus main body.
2. Related Background Art
Defective images such as low optical density images and/or lost images can appear when an electrophotographic image forming apparatus, which may be a laser printer, becomes short of developer (e.g., toner) in operation. Therefore, conventionally, the amount of residual developer (toner) is detected normally in the developing apparatus. Existing electrophotographic image forming apparatus are equipped with means for displaying an indication that or a warning that there is a toner shortage, which means is operated when such a shortage arises so that toner may be supplied (or replenished) before defective images appear.
For the purpose of supplying toner, the developer containing portion of the image forming apparatus that contains toner is realized in the form of a cartridge so that toner may be supplied by replacing the cartridge. Additionally, an arrangement of combining the developer containing portion that also operates as a developing apparatus and the electrophotographic photosensitive member that is an image bearing member and putting them into a cartridge, which is referred to as a process cartridge and detachably mountable to the image forming apparatus, is being used popularly.
As means for detecting the amount of residual developer for detecting the amount of residual toner in the developing apparatus, those of the electrostatic capacity detection type are known (see, Japanese Patent Application Laid-open No. 2000-206774).
FIG. 14 of the accompanying drawings schematically illustrates a developing apparatus equipped with a means for detecting the amount of residual developer of the electrostatic capacity detection type.
Referring to FIG. 14, the developing apparatus 4 comprises a developing sleeve 43 that is an electroconductive cylindrical member adapted to operate as developer carrying member. A plate antenna PA, which is an electroconductive detecting member, is arranged in developer containing portion 40 with toner interposed between itself and the developing sleeve 43. When detecting the amount of residual toner in the developing apparatus 4, the developing sleeve 43 and the plate antenna PA are operated as electrodes and the change in the electrostatic capacity between the two electrodes, or the developing sleeve 43 and the plate antenna PA, that is observed when a developing bias is applied to the developing sleeve 43 represents the change in the amount of residual toner existing between the developing sleeve 43 and the plate antenna PA in order to detect the amount of residual toner.
The electrostatic capacity between the developing sleeve 43 and the plate antenna PA is observed by applying a developing bias, that is an oscillating voltage formed by superimposing an AC voltage on a DC voltage, to the developing sleeve 43 from a developing bias source (not shown) and converting the electric current flowing between the plate antenna PA and the ground into a DC voltage by means of a detection circuit. In other words, the electrostatic capacity is read as the value of the induced voltage that is generated between the two electrodes, which are the developing sleeve 43 and the plate antenna PA.
However, the value of the developing bias that is applied to the developer carrying member, which is typically a developing sleeve 43 as shown in FIG. 14, is more often than not selected so as to be able to achieve the best image quality even when the developing bias is used in order to detect the amount of residual toner in the developing apparatus.
On the other hand, the developing apparatus is arranged in the process cartridge and, in many cases, the latter is replaced by a new one when toner is in short supply. If the developing bias that is held to a single constant value is used to detect the amount of residual toner in the developing apparatus, it has become increasingly difficult to accurately detect the amount of residual toner in order to achieve the best image quality all the way from the very start to the end of the service life of the process cartridge because of the current trend of prolonged service life of process cartridges.
In view of the above, Japanese Patent Application Laid-open No. 2002-244365 proposes a technique of defining the developing bias according to the environment and the amount of residual toner in an image forming apparatus in order to achieve the best image quality all the way from the very start to the end of the service life of the process cartridge. With the proposed technique, it is possible to constantly produce a high quality image by appropriately switching the amplitude of the AC voltage, which forms the developing bias with a DC voltage, or reducing the amplitude of the developing bias as a function of the time spent in service to be more accurate.
It has been found that the optical density of the produced image falls remarkably particularly in a latter half of the service life of the process cartridge when it is put to use in a hot and highly humid environment. With the proposed technique, the problem of a defective image with a low optical density may be dissolved by raising the amplitude of the AC voltage of the developing bias.
However, if the amplitude of the AC voltage of the developing bias is raised in a cold and low humidity environment and/or from the initial stages of the service life of the process cartridge, particles of developer can be driven to fly and land on a non-image area where no image is supposed to be formed to produce a defective image as shown in FIG. 15A relative to a normal image shown in FIG. 15B. Such a phenomenon is referred to as “fog”.
Thus, the measure to be taken for achieving the best image quality may be to select an appropriate value for the amplitude of the AC voltage depending on the environment in such a way that a small value is used for the AC voltage in a cool and low humidity environment or in an environment that is hot and humid but no fog would occur particularly or in the initial stages of the service life of the process cartridge but a large value is used for the AC voltage in the latter stages of the service life of the process cartridge. In other words, the amplitude of the AC voltage of the developing bias needs to be switched depending on the environment and the amount of residual toner in the developing apparatus.
As such a measure, Japanese Patent Application Laid-open No. 2003-307994 discloses a technique of providing two image forming modes including the first image forming mode where the amplitude of the AC voltage of the developing bias is defined for the initial stages of the service life of the process cartridge so as to solve the problem of degraded images, such as foggy images, and realize the best image quality printing and the second image forming mode where the amplitude of the AC voltage of the developing bias is so defined as to prevent images of low optical density from being produced when the amount of residual toner is reduced in a latter half of the service life of the process cartridge. The first image forming mode is switched to the second image forming mode at the middle of the service life of the cartridge.
However, the above-described arrangement for selecting the amplitude of the AC voltage of the developing bias out of a plurality of values depending on the selected image forming mode entails problems including that the area and the cost required for necessary circuits will be significant and that the output value of the circuit for detecting the amount of residual toner, or the means for detecting the amount of residual toner, by means of the developing bias can show discrepancies, thereby remarkably lowering the accuracy of the result of residual toner detecting process.
For instance, assume an arrangement where two image forming modes are provided so as to be switched from one to the other as a function of the surrounding environment and the amount of residual toner in the developing apparatus. Then, if the amplitude of the AC voltage of the developing bias before a switch is 2.0 KV and the amplitude of the AC voltage of the developing bias after the switch is 2.5 KV, the relationship between the amount of residual toner and the detection output value on the amount of residual toner is typically expressed by the solid line in the graph of FIG. 5, where the detection output value on the amount of residual toner changes abruptly when the image forming mode is switched from one to the other.
Referring to FIG. 5, 3 V is selected for the output voltage at which a toner shortage is judged to take place because defective images are produced owing to shortage of toner when the detection output value on the amount of residual toner is equal to or greater than 3 V while the amplitude of the AC voltage of the developing bias is 2.0 KV. If the amplitude of the AC voltage of the developing bias is switched from 2.0 KV to 2.5 KV when the amount of residual toner is equal to or less than 20% of the capacity in a hot and highly humid environment of the image forming apparatus, the detection output value on the amount of residual toner falls from 2.45 V to 2.1 V so that the detection output value on the amount of residual toner will be less than 3 V if the amount of residual toner is 0% of the capacity. In other words, no toner shortage is judged to take place although there is no toner left. Then, the image forming operation will be continued in a state of toner shortage to consequently give rise to defective images.
In short, with the above-described arrangement of detecting the amount of residual developer in the developing apparatus by way of the dielectric voltage that is generated between the electrodes arranged in the developer containing portion of the developing apparatus by the developing bias formed by laying an AC voltage on a DC voltage so as to be applied to the developer carrying member, there arise a problem that the amount of residual toner in the developing apparatus can be detected falsely due to the discrepancy between the detected values for the amount of residual developer before and after a switch of the amplitude of the AC component of the developing bias.